The present invention generally pertains to video detector circuits for radar systems and is particularly directed to enhancing the signal-to-noise ratio and target-to-clutter ratio in a received radar signal prior to signal processing.
A typical video detector circuit essentially includes a detector diode having one of its electrodes connected to an input terminal for detecting an applied RF signal, and its other electrode connected to a load terminal for providing a detected signal; and a load resistance connected between the load terminal and a bias terminal. The operating characteristics of the diode are defined by a curve having a gradual portion in which the diode forward current increases slightly as the diode forward voltage is increased from zero volts, a steep portion in which the diode forward current increases sharply as the diode forward voltage is further increased, and a knee portion defining a transition between the gradual and steep portions. A typical characteristic curve for a diode is shown in FIG. 1.
The applied RF signal typically contains an rms voltage caused by thermal noise and components representative of clutter in addition to target information. Clutter is a term used to describe confusing and unwanted echoes which interfere with detection of the desired target information. Typically clutter may be caused by the ground and by trees, brush and other ground vegetation. The quiescent operating point of the detector diode serves as a dc pedestal level for the target, clutter and thermal noise voltage components which are contained in the applied RF signal.
Typically the bias terminal is at circuit ground potential; and the quiescent operating point of the detector diode is determined by the extent to which the diode is forward biased by the energy content of the applied RF signal. Such a video detector circuit is said to be self-biased. In a self-biased video detector circuit the quiescent operating point of the detector diode is in the steep portion of its characteristic curve. Accordingly when an applied signal which contains an rms noise voltage within a predetermined range is provided across the diode detector in a self-biased video detector circuit, the resultant diode rms noise current is in a range about the quiescent operating point in the steep portion of the characteristic curve, thereby causing the signal-to-noise ratio of the detected signal to be close in value to the signal-to-noise ratio of the applied signal.